Japanese Patent-laid-open 2000-230526 discloses a structure of a cross-shaped groove in which each pair of inner side faces of the first to fourth wing grooves forming the cross-shaped groove are parallel except at distal end portions thereof, the distal end portions are inclined outwardly sideward, and all inner side faces of the first to fourth wing grooves are vertical.
As all inner side faces of the first to fourth wing grooves are vertical faces, it may be said that coming-off caused by upward sliding of a driver bit (i.e., upward slipping of a driver bit) during rotation hardly occurs. However, interlocking effect when the cross-shaped hole is fit with a driver bit is insufficient to cause a problem that the screw drops off from the driver bit.
Further, there is a fear of occurrence of misalignment of centering of the screw against a pilot hole and non-smooth operation of screw alignment against a pilot hole entry with the driver bit to eventually cause an oblique screwing problem due to excessive clearance between the first to fourth wing teeth of the driver bit and every vertical inner side face of the first to fourth wing grooves.